Effects of different fertilization measures on acidified yellow-red soil improvement and agronomic characters of rapeseed

The acidification of yellow-red soil readily induces soil fertility degradation. While inorganic mineral amendments often lead to soil compaction, organic fertilizers exhibit slow improvement effects. As an alkaline organic liquid fertilizer, the efficacy of biogas slurry combined with organic fertilizer in ameliorating acidified yellow-red soil remains unclear. This study comparatively investigated the effects of different amendment measures on rape yield and soil improvement, evaluating the performance of biogas slurry-organic fertilizer co-application based on soil amelioration outcomes and rape agronomic traits. The experiment included five treatments: no fertilization (CK), conventional chemical fertilization (NPK), NPK plus organic fertilizer (M), NPK plus organic fertilizer and hydrated lime (M+L), and biogas slurry substituting chemical fertilizers (with equivalent NPK) plus organic fertilizer (M+B). Comprehensive analyses were conducted on soil acidity indicators, organic matter content, nutrient status, organic/inorganic phosphorus fractions, ¹³C-NMR spectra, enzyme activities, as well as rape yield and nutrient content under different treatments. Pearson correlation analysis was performed to determine the optimal amendment strategy. The results demonstrated that: 1) Compared with NPK treatment, all organic fertilizer treatments significantly increased rape yield and nutrient content (P<0.05). Both M+L and M+B treatments substantially elevated soil pH by 2.36 and 1.42, respectively, while reducing exchangeable aluminum, exchangeable hydrogen, and total exchangeable acidity by 99.68%/92.31%/99.41% (M+L) and 92.89%/52.07%/91.37% (M+B) (P<0.05). The M+B treatment significantly enhanced soil organic matter, available nitrogen, and available potassium by 15.47%, 6.10%, and 148.07% (P<0.05), though total nitrogen and potassium contents showed no significant improvement (P>0.05). 2) Organic fertilizer treatments significantly increased total phosphorus (61.15%-136.00%), inorganic phosphorus (304.65%-500.00%), and available phosphorus (7.46-28.52 fold) compared to NPK (P<0.05), with M+B showing the most pronounced effects. The M+B treatment also significantly improved alkaline phosphatase, neutral phosphatase, and acid phosphatase activities by 50.56%, 16.00%, and 43.35%, respectively, whereas M and M+L treatments significantly suppressed all three phosphatase activities (P<0.05). 3) ¹³C-NMR analysis revealed that organic fertilizer treatments decreased the proportions of alkyl-C and carbonyl-C while increasing O-alkyl-C and aromatic-C fractions. Among M, M+L, and M+B treatments, the alkyl-C/O-alkyl-C ratio followed M+B > M+L > M, while the aliphatic-C/aromatic-C ratio exhibited M > M+L > M+B. These findings indicate that the combined application of organic fertilizer and biogas slurry effectively mitigates soil acidification, enhances phosphorus availability, improves organic matter stability, and promotes nutrient uptake by rape, representing a sustainable approach for ecological restoration of acidified yellow-red soils.